The invention relates to a circuit arrangement for the measurement of air mass employing the hot-wire anemometer principle, in particular for motor vehicles with an internal combustion engine.
A circuit arrangement of this kind is known from the publication DE-OS 38 02 422 and is shown in principle in FIG. 3. This known circuit arrangement consists of an electrical bridge with two bridge arms. One of these arms is an air flow measurement resistor R.sub.L around which the air flows and connected in series with a current measurement resistor R.sub.1. The other arm is a temperature compensation resistor R.sub.T that detects the air temperature and is followed by a correction resistor R.sub.K connected in series with a fixed resistor R.sub.2. The air flow measurement resistor R.sub.L and the temperature compensation resistor R.sub.T are, for example, hot-film sensors with positive temperature coefficients with identical temperature characteristics. The difference voltage in the bridge diagonal branch is picked up by a differential amplifier U.sub.1 and serves to balance the bridge.
The current flowing through the current measurement resistor R.sub.1 after the bridge has been balanced, or the voltage U.sub.M across the current measurement resistor R.sub.1 after the bridge has been balanced, is taken as measurement for the momentary air mass flow.
The measuring bridge is always regulated with the help of the differential amplifier U.sub.1 such that the bridge is balanced for every air flow and temperature state, i.e., the differential amplifier U.sub.1 varies the current through the air flow measurement resistor R.sub.L until the bridge difference voltage measured by the differential amplifier U.sub.1 becomes zero. In so doing, the air flow measurement resistor R.sub.L heats up and its resistance value changes in accordance with its temperature characteristics.
The circuit represents a constant resistance regulator in which the resistance of the air flow measurement resistor R.sub.L behaves in accordance with the following formula: EQU R.sub.L (T)=(R.sub.T (T)+R.sub.K).multidot.R.sub.1 /R.sub.2 ( 1)
When the bridge is in the steady state according to FIG. 3, the power P.sub.se supplied to the air flow measurement resistor R.sub.L is just as great as the power output from this air flow measurement resistor R.sub.L to the fluid flowing by. The resistance values in the bridge are so selected that a preferably constant temperature of approximately 130.degree. K. above that of the fluid flowing past settles in. A change in the value of the fixed resistor R.sub.2 causes a change in the overtemperature level, while changes to the correction resistor R.sub.K influence the effects of the temperature sensing with the temperature compensation resistor R.sub.T. The correction resistor R.sub.K thus determines whether, when the fluid temperature rises, the over temperature also rises, falls or preferably remains constant on account of the shorter response time when there is a change in temperature.
For the current I.sub.se flowing through the air flow measurement resistor R.sub.L, the following formula applies: EQU I.sub.se =K.sub.0 .multidot.(P.sub.se /R.sub.L).sup.1/2 ( 2)
where K.sub.0 is a constant. The output voltage U.sub.m, which represents a measure for the air flow, is a function of the mass flow (dm/dt) in accordance with King's Law as defined by the following formula: EQU U.sub.M =K.sub.1 +K.sub.2 .multidot.(dm/dt).sup.1/4 ( 3)
where K.sub.1 and K.sub.2 are constants.
The primary measured quantity of a hot-wire, or hot-film, anemometer is the power converted at the air flow measurement resistor R.sub.L. Since the temperature compensation resistance R.sub.T varies with the temperature of the fluid, according to equation (1) above the resistance value of the air flow measurement resistor R.sub.L will also vary, so that a constant resistance regulator is provided basically only for discrete temperatures. In order to detect the power converted at the air flow measurement resistor R.sub.L, it is therefore actually necessary to detect the power P.sub.se converted at this resistor. Normally, however, only the current I.sub.se is evaluated by the current measurement resistor R.sub.1, which means that an error always results in determining the power. Since the air mass flow must always be acquired as a measured quantity, however, this error is almost compensated because calculations of the heat transfer coefficient .alpha. show that for the same mass flow the power that can be converted at the air flow measurement resistor R.sub.L rises with temperature. The resistance value of the air flow measurement resistor R.sub.L also increases with rising temperature so that the current I.sub.se that settles in is in fact approximately constant (King's Law). However, there remains a residual gradient with respect to temperature that causes an error of between 2 and 5% referred to the measured value in the temperature range that is of interest for motor vehicle applications.